In a mechanical structural part used for various industrial machines such as the transportation equipment, construction equipment, other industrial machines and the like particularly for the raw material of the mechanical structural part requiring high strength, a low-alloyed steel for machine structural use (case hardening steel) stipulated in the JIS standards such as SCr, SCM, SNCM and the like has been conventionally used. The case hardening steel is formed into a desired part shape by mechanical work such as forging, machining and the like, is thereafter subjected to surface hardening treatment (case hardening treatment) such as carburizing, carbonitriding and the like and thereafter goes through the steps such as polishing and the like, and the mechanical structural part is manufactured.
In recent years, in the manufacturing step of the mechanical structural part, change from conventional hot forging and warm forging to cold forging has been desired. Cold forging is working executed in the atmosphere of 200° C. or below normally, and cold forging has such advantages that the productivity is excellent and both of the dimensional accuracy and the yield of the steel are excellent compared to those in hot forging and warm forging. However, when the case hardening steel stipulated in the JIS standards described above is used, such problems occur as insufficient cold forgeability, deterioration of the mechanical properties such as the part strength because of coarsening of the crystal grain by carburizing after cold forging. Therefore, as a technology for preventing coarsening of the crystal grain, technologies of Patent Literatures 1-3 have been disclosed. In these literatures, technologies of adding elements such as Ti, Nb, and the like, finely dispersing precipitates such as TiC, Nb(CN) and the like into steel thereby exerting a pinning effect, and preventing coarsening of the crystal grain have been disclosed. Also, in Patent Literature 4 for example, a technology for improving the cold forgeability by adjusting the adding amount of alloy elements while taking such crystal grain coarsening preventing measures has been proposed.